High density lipoproteins (HDL) their major protein apoA-I directly protect against atherosclerosis cardiovascular disease, but the mechanisms are not well understood. In addition to its role in reverse cholesterol transport, HDL may have other properties that contribute to its anti-atherogenic effects. We hypothesize that HDL has anti oxidant and anti-inflammatory effects that are relevant and operative in vivo and are important components of the atheroprotective effects of HDL. The focus of this proposal is to test this hypothesis using relevant in vivo models in mice and in humans. We will utilize approaches as described in detail in Project 1 (oxidatively modified lipids), Project 2 (nitrated proteins), Project 3 (oxidatively modified extracellular matrix) and Project 4 (oxidative DNA adducts) of this SCOR proposal, to study novel mechanisms by which HDL inhibits atherogenesis. Specific Aim 1: To test the hypothesis that HDL inhibits oxidation of lipids (lipoprotein and plasma membrane), nitration of proteins, oxidative fragmentation of extracellular matrix, and DNA in vitro and to determine the mechanism for these anti-oxidant effects of HDL. Specific Aim 2: To use mouse models of atherosclerosis to determine the effect of both apoA-I deficiency and apoA-1 overexpression on markers of oxidative stress and inflammation in vivo and the relationship to atherosclerosis. Specifically, we will test: 1) that apoA-1 deficiency accelerates the age-related progression of markers of oxidant stress, vascular inflammation, and atherosclerosis; 2) that apoA-I overexpression reduces in vivo oxidant stress and vascular inflammation; 3) that HDL?s effect in increasing prostacyclin is an important in vivo by determining the effect of overexpression of apoA-I on atherosclerosis in the prostacyclin receptor (IP) knockout mouse; and 4) that HDL inhibits atherosclerosis even in the setting of impaired reverse cholesterol transport by determining the effect of apoA-I overexpression on atherosclerosis in SR-BI knockout mice. Specific Aim 3: A prospective observational study will be performed in asymptomatic persons with high HDL-C levels and two comparison groups: individuals with average HDL-C levels and individuals with low HDL-C levels. Baseline markers of oxidant stress and inflammation will be compared among the three groups. In addition, two quantitative measures of subclinical atherosclerosis, carotid intimal-medial thickness (IMT) and coronary artery calcification (CAC) will be determined at baseline. These baseline measures will be repeated at two years. In the three HDL strata, baseline levels of oxidant stress and inflammation will be examined as predictors of progression of atherosclerosis. The experiments proposed will generate substantial insight into the mechanisms by which HDL protects against atherosclerosis, information crucial to the rational development of novel HDL-raising strategies.